1. Field of the Invention
This invention relates to the thermal cracking of hydrocarbons using a pyrolysis furnace having a convection section that contains at least one conduit that carries bend fittings and hard facing in at least one of the bend fittings. This invention is especially useful in processes that utilize, as a primary (initial) feedstock for an olefin production plant, heavier hydrocarbonaceous materials that contain solid particles and/or tend to form coke in the convection section of a pyrolysis furnace, particularly, but not exclusively, whole crude oil and/or natural gas condensate feeds.
2. Description of the Prior Art
Thermal cracking (pyrolysis) of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, a hydrocarbon containing feedstock is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. The steam/hydrocarbon mixture is preheated in the convection zone of the furnace to from about 900 to about 1,000 degrees Fahrenheit (F), and then enters the reaction (radiant) zone where it is very quickly heated to a severe hydrocarbon thermal cracking temperature in the range of from about 1,450 to about 1,550 F. Thermal cracking is accomplished without the aid of any catalyst.
This process is carried out in a pyrolysis furnace (steam cracker) at pressures in the reaction zone ranging from about 10 to about 30 psig. Pyrolysis furnaces have internally thereof a convection section (zone) and a separate radiant section (zone). Preheating functions are primarily accomplished in the convection section, while severe cracking mostly occurs in the radiant section.
After thermal cracking, depending on the nature of the primary feed to the pyrolysis furnace, the effluent from that furnace can contain gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. These gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic, alicyclics, and/or aromatic. The cracked gas can also contain significant amounts of molecular hydrogen (hydrogen).
The cracked product is then further processed in the olefin production plant to produce, as products of the plant, various separate individual streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule, fuel oil, and pyrolysis gasoline. Each separate individual stream aforesaid is a valuable commercial product in its own right. Thus, an olefin production plant currently takes a part (fraction) of a whole crude stream or condensate, and generates therefrom a plurality of separate, valuable products.
Thermal cracking came into use in 1913 and was first applied to gaseous ethane as the primary feed to the cracking furnace for the purpose of making ethylene. Since that time the industry has evolved to using heavier and more complex hydrocarbonaceous gaseous and/or liquid feeds as the primary feed for the cracking furnace. Such feeds can now employ a fraction of whole crude or condensate which is essentially totally vaporized while thermally cracking same. The cracked product can contain, for example, about 1 weight percent (wt. %) hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, all wt. % being based on the total weight of that product, with the remainder consisting mostly of other hydrocarbon molecules having from 4 to 35 carbon atoms per molecule.
Natural gas and whole crude oil(s) were formed naturally in a number of subterranean geologic formations (formations) of widely varying porosities. Many of these formations were capped by impervious layers of rock. Natural gas and whole crude oil (crude oil) also accumulated in various stratigraphic traps below the earth's surface. Vast amounts of both natural gas and/or crude oil were thus collected to form hydrocarbon bearing formations at varying depths below the earth's surface. Much of this natural gas was in close physical contact with crude oil, and, therefore, absorbed a number of lighter molecules from the crude oil.
When a well bore is drilled into the earth and pierces one or more of such hydrocarbon bearing formations, natural gas and/or crude oil can be recovered through that well bore to the earth's surface.
The terms “whole crude oil” and “crude oil” as used herein means liquid (at normally prevailing conditions of temperature and pressure at the earth's surface) crude oil as it issues from a wellhead separate from any natural gas that may be present, and excepting any treatment such crude oil may receive to render it acceptable for transport to a crude oil refinery and/or conventional distillation in such a refinery. This treatment would include such steps as desalting. Thus, it is crude oil that is suitable for distillation or other fractionation in a refinery, but which has not undergone any such distillation or fractionation. It could include, but does not necessarily always include, non-boiling entities such as asphaltenes or tar. As such, it is difficult if not impossible to provide a boiling range for whole crude oil. Accordingly, whole crude oil could be one or more crude oils straight from an oil field pipeline and/or conventional crude oil storage facility, as availability dictates, without any prior fractionation thereof.
Natural gas, like crude oil, can vary widely in its composition as produced to the earth's surface, but generally contains a significant amount, most often a major amount, i.e., greater than about 50 weight percent (wt. %), methane. Natural gas often also carries minor amounts (less than about 50 wt. %), often less than about 20 wt. %, of one or more of ethane, propane, butane, nitrogen, carbon dioxide, hydrogen sulfide, and the like. Many, but not all, natural gas streams as produced from the earth can contain minor amounts (less than about 50 wt. %), often less than about 20 wt. %, of hydrocarbons having from 5 to 12, inclusive, carbon atoms per molecule (C5 to C12) that are not normally gaseous at generally prevailing ambient atmospheric conditions of temperature and pressure at the earth's surface, and that can condense out of the natural gas once it is produced to the earth's surface. All wt. % are based on the total weight of the natural gas stream in question.
When various natural gas streams are produced to the earth's surface, a hydrocarbon composition often naturally condenses out of the thus produced natural gas stream under the then prevailing conditions of temperature and pressure at the earth's surface where that stream is collected. There is thus produced a normally liquid hydrocarbonaceous condensate separate from the normally gaseous natural gas under the same prevailing conditions. The normally gaseous natural gas can contain methane, ethane, propane, and butane. The normally liquid hydrocarbon fraction that condenses from the produced natural gas stream is generally referred to as “condensate,” and generally contains molecules heavier than butane (C5 to about C20 or slightly higher). After separation from the produced natural gas, this liquid condensate fraction is processed separately from the remaining gaseous fraction that is normally referred to as natural gas.
Thus, condensate recovered from a natural gas stream as first produced to the earth's surface is not the exact same material, composition wise, as natural gas (primarily methane). Neither is it the same material, composition wise, as crude oil. Condensate occupies a niche between normally gaseous natural gas and normally liquid whole crude oil. Condensate contains hydrocarbons heavier than normally gaseous natural gas, and a range of hydrocarbons that are at the lightest end of whole crude oil.
Condensate, unlike crude oil, can be characterized by way of its boiling point range. Condensates normally boil in the range of from about 100 to about 650 F. With this boiling range, condensates contain a wide variety of hydrocarbonaceous materials. These materials can include compounds that make up fractions that are commonly referred to as naphtha, kerosene, diesel fuel(s), and gas oil (fuel oil, furnace oil, heating oil, and the like). Naphtha and associated lighter boiling materials (naphtha) are in the C5 to C10, inclusive, range, and are the lightest boiling range fractions in condensate, boiling in the range of from about 100 to about 400 F. Petroleum middle distillates (kerosene, diesel, atmospheric gas oil) are generally in the C10 to about C20 or slightly higher range, and generally boil, in their majority, in the range of from about 350 to about 650 F. They are, individually and collectively, referred to herein as “distillate” or “distillates.” Distillate compositions can have a boiling point lower than 350 F and/or higher than 650 F, and such distillates are included in the 350-650 F range aforesaid, and in this invention.
The olefin production industry is now progressing beyond the use of fractions of crude oil or condensate (gaseous and/or liquid) as the primary feed for a cracking furnace to the use of whole crude oil and/or condensate itself.
Recently, U.S. Pat. No. 6,743,961 (hereafter “U.S. Pat. No. '961”) issued to Donald H. Powers. This patent relates to cracking whole crude oil by employing a vaporization/mild cracking zone that contains packing. This zone is operated in a manner such that the liquid phase of the whole crude that has not already been vaporized is held in that zone until cracking/vaporization of the more tenacious hydrocarbon liquid components is maximized. This allows only a minimum of solid residue formation which residue remains behind as a deposit on the packing. This residue is later burned off the packing by conventional steam air decoking, ideally during the normal furnace decoking cycle, see column 7, lines 50-58 of that patent. Thus, the second zone 9 of that patent serves as a trap for components, including hydrocarbonaceous materials, of the crude oil feed that cannot be cracked or vaporized under the conditions employed in the process, see column 8, lines 60-64 of that patent.
Still more recently, U.S. Pat. No. 7,019,187 issued to Donald H. Powers. This patent is directed to the process disclosed in U.S. Pat. No. '961, but employs a mildly acidic cracking catalyst to drive the overall function of the vaporization/mild cracking unit more toward the mild cracking end of the vaporization (without prior mild cracking)—mild cracking (followed by vaporization) spectrum.
The disclosures of the foregoing patents, in their entirety, are incorporated herein by reference.
Coke, as used herein, means a high molecular weight carbonaceous solid, and includes compounds formed from the condensation of polynuclear aromatics. Coke has heretofore been found to be formed essentially only in the radiant section of furnaces where the primary cracking of the furnace feed occurs.
Heretofore, little or no coke was found in the convection sections of cracking furnaces, and it was common thought in the industry that erosion protection in the convection section of furnaces was not necessary. However, it has been found that as the industry has progressed to the cracking of heavier and more complex feeds, the tendency to form coke in the convection section of a furnace has increased. This tendency to form coke in the convection section of a furnace, because of its primarily preheating function, was unexpected. Further, this coke formation tendency can increase even more as the industry moves toward using whole crude oil and/or condensate as a primary furnace feed.
It has recently been found that with an increased tendency to (1) form coke in the convection section of a furnace and (2) carry solid particles such as sand and asphaltenes into the furnace by way of heavier feedstocks, erosion of fittings in the convection section tubing, cross-over line, and/or transfer line has increased. For example, in a convection section conduit (line), cracking feed can pass there through at a velocity of from about 250 to about 700 feet per second. This high velocity cracking feed carrying solid particles and/or coke particles can cause substantial erosion of convection section conduits, particularly when a stream of feed carrying such particles is changed in its direction of flow by impinging same on a 90 degree elbow fitting, a 180 degree u-tube fitting, and the like, as explained in greater detail hereinafter.
This invention addresses the unexpected problem of erosion in the convection section of cracking furnaces as well as crossover lines and transfer lines.